"Petzl" wrote
As cat is doing this in a mooching exercise you need to whack it one
between the eyes

A whacking tool is something that won't hurt cat but make a fair
noise. A good one is the soft hollow cardboard roll from the center of
a Paper Tower or cling wrap roll.

It's not the force of the whack that counts, it's the noise, you can
also do the loud NO same time.

Right. It is never necessary, and it is never, ever advisable to hit a cat.